Implantable cardioverter-defibrillator (ICD) art has long distinguished ventricular tachyarrhythmias by rate and type. Ventricular tachycardias (VTs), which generally include arrhythmias having rates between 150 and 250 bpm or more, can be further differentiated by their ECG configuration as either monomorphic or polymorphic. Arrhythmias with rates above an upper VT range, and up to approximately 350 bpm, are often termed ventricular flutter waves. Chaotic waveforms at rates higher than 350 bpm are classified as ventricular fibrillation (VF).
To treat each type of arrhythmia with an appropriate therapy, ICDs have been equipped with “tiered therapies”. Such devices are generally referred to as Pacer-Cardioverter-Defibrillators (PCDs). PCDs generally differentiate arrhythmias by rates, with programmable therapies to treat a respective type of detected arrhythmia(s). In such devices, the less-dangerous arrhythmias such as VT are treated by delivering a series of low-power pacing pulses to the heart at a relatively high rate. This therapy is often referred to as anti-tachyarrhythmia pacing therapy (ATP). In contrast, more perilous arrhythmias such as VF are often treated using a more aggressive shock therapy. For example, many PCDs may be programmed to first treat a VT with low-power ATP and then, if the VT progresses to ventricular flutter or fibrillation, deliver one or more high-power cardioversion or defibrillation shocks.
Many implantable anti-tachycardia pacemakers have the capability of providing a variety of anti-tachycardia pacing regimens. Normally, these regimens are applied according to a pre-programmed sequence, such as burst or ramp therapies among others. Each therapy extends over a series of a predetermined number of pacing pulses. After the series of pacing pulses is delivered, the devices check to determine whether the series of pulses was effective in terminating the detected tachyarrhythmia. Termination is generally confirmed by a return to sinus rhythm, for example, identified by a sequence of a predetermined number of spontaneous depolarizations separated by greater than a defined interval. In the absence of detected termination, the PCD applies more aggressive therapies such as synchronized cardioversion shocks or defibrillation shocks. While the delivery of ATP in some cases makes shock therapy unnecessary, a further reduction in the frequency of shock delivery is still desirable.
Applying an electrical pulse to the heart, whether a pacing pulse or a shock, requires charging of one or more output capacitors. Generally, the amount of energy required to delivery pacing pulses is low. This type of therapy may therefore be delivered by a low-power output circuit relatively instantaneously. On the other hand, high-power shocks require a set of high-voltage capacitors that may require several seconds to reach a fully-programmed charge. As stated above, when a tiered therapy approach is utilized, both of these therapies may be used to “break” the tachyarrhythmia. That is, first ATP is delivered. During this time, the high-voltage capacitors may be charged so that if ATP fails to break the VT, a high-voltage shock may be delivered soon thereafter. If the VT is terminated by ATP, the charged high-voltage capacitors must abort delivery and internally “leak off” the stored energy in the capacitors, which depletes battery power. This can significantly shorten the useful life of the implanted device.
What is needed, therefore, is a method and apparatus to deliver successful ATP therapy without needlessly depleting battery resources.